This invention relates to a nuclear reactor containment arrangement wherein a permanent cavity seal ring extends across an annular expansion gap between a reactor pressure vessel and a containment wall to provide a water tight seal therebetween.
A nuclear reactor pressure vessel is typically positioned in a cavity or containment system formed of concrete and shielding material. The reactor pressure vessel is spaced from the containment wall so as to provide an annular expansion gap and permit the vessel to expand in both a horizontal and vertical direction. An upper annular portion above the vessel serves as a refueling canal. The refueling canal, while maintained dry during reactor operation, is filled with water during refueling of the nuclear reactor. In refueling, the head of the reactor pressure vessel is removed and the refueling canal is filled with borated water and provides adequate shielding in order to maintain radiation levels within acceptable limits.
The reactor pressure vessel has flanges which secure the head assembly to the body of the vessel, with a lower flange on the peripheral wall of the vessel ear the vessel opening that is sealable to the containment wall. The seal between the reactor pressure vessel and the containment wall serves as a floor for the body of water that is placed in the refueling canal.
It has previously been proposed to use removable seals to seal the vessel to the containment wall, such as removable gasket-type seal rings, but such elastomeric gaskets are susceptible to degradation and leakage and require inordinate installation time and effort.
During normal reactor operation with a removable seal, the annular expansion gap between the reactor pressure vessel and the containment wall is covered with portable thermal insulation and a large ring-shaped water tank for neutron shielding. These large components must be removed for refueling operations and later reinstalled. Since the concrete structure of the containment wall, on the outside of the annular expansion gap, must be kept below design temperature limits, cooling air is forced up between the face of the concrete liner in the cavity and out through openings in the supports of the neutron shield tanks during operation of the reactor. In addition, any personnel access for instrumentation service and reactor vessel nozzle inspection is normally only available during the period when all of the hardware is removed from the top of the annular expansion gap, i.e. the removable seal, shielding and insulation.
In order to solve the problems associated with temporary or removable seals, the installation of permanent seals, such as that described in U.S. Pat. No. 4,747,993, to Meuschke et al., assigned to the assignee of the present invention, and the contents of which are incorporated by reference herein, were developed. The permanent seal ring described in U.S. Pat. No. 4,747,993 has an annular ring plate which is formed as a step-shaped cross-section, having a first annular portion that is fixed to a mounting plate on the containment wall, a cylindrical portion, and a second annular portion which extends horizontally past, and is spaced from, the outer edge of the flange on the reactor pressure vessel. An L-shaped flexure member is affixed to the second annular portion and has one leg thereof secured to the second annular portion and the other leg which extends to and sealingly engages the flange. The U.S. Pat. No. 4,747,993 type of permanent sealing ring, while having superior properties over prior art rings, is designed to have a backup member, in addition to the sealing flexure member, in the event that the seal between the other leg and the flange is disturbed by deflective or other forces. This backup member, which is a backup plate and flashing, provides a backup structure for the flexure member to preclude the possibility of major leakage of shielding water in the event of structural failure of the flexural member. While the backup member does not form a water-tight seal it does function as a flow restrictor to prevent a catastrophically sudden and/or complete loss of shielding water from the refueling canal during refueling operations. The support arms of this annular ring seal also use leveling bolts which engage the flange for initially aligning the ring during installation and providing auxiliary support for the annular ring seal during the refueling operation.
It is an object of the present invention to provide a permanent cavity seal ring for a nuclear reactor containment arrangement as a single structure which results in water-tight sealing, neutron shielding, thermal insulation, containment wall cooling and service access to the annular expansion gap.